In a conventional screw machine, a male rotor and a female rotor, disposed in respective parallel overlapping bores defined within a rotor housing, coact to trap and compress volumes of gas. While two rotors are the most common design, three, or more, rotors may coact in pairs. The male and female rotors differ in their lobe profiles and in the number of lobes and flutes. For example, the female rotor may have six lobes separated by six flutes, the while conjugate male rotor may have five lobes separated by five flutes. Accordingly, each possible combination of lobe and flute coaction between the rotors occurs on a cyclic basis. The coaction between the conjugate pairs of rotors is a combination of sliding and rolling contact which can produce different rates of wear. In addition to coacting in pairs, the rotors coact as well with the housing. Because all combinations of rotor contact takes place between conjugate pairs, the sealing/leakage between the various combinations may be different due to manufacturing tolerances and wear patterns. This can be the case even though manufacturing tolerances are held very tight with the attendant manufacturing costs and adequate lubrication or other liquid injection is provided for sealing.
The profile design of conjugate pairs of screw rotors must be provided with a clearance in most sections. The need to provide a clearance is the result of a number of factors including: thermal growth of the rotors as a result of gas being heated in the compression process; deflection of the rotors due to pressure loading resulting from the compression process; tolerances in the support bearing structure and machining tolerances on the rotors which may sometimes tend to locate the rotors too close to one another which can lead to interference; and machining tolerances on the rotor profiles themselves which can also lead to interference. Superimposed upon these factors is the existence of pressure and thermal gradients as the pressure and temperature increase in going from suction to discharge. The pressure gradient is normally in one direction during operation such that fluid pressure tends to force the rotors towards the suction side. The rotors are conventionally mounted in bearings at each end so as to provide both radial and axial restraint. The end clearance of the rotors at the discharge side is critical to sealing and the fluid pressure tends to force open the clearance.
There are certain sections of the rotor, such as the contact band, where zero clearance is maintained between the rotors. The segment of the rotor defining the contact band is the region where the required torque is transmitted between the rotors. The load between the rotors is different for a male rotor drive and for a female rotor drive. In a male drive the loading between the rotors may be equivalent to about 10% of the total compressor torque, whereas in the case of female rotor drive the loading between the rotors may be equivalent to about 90% of the total compressor torque. These segments are conventionally positioned near the pitch circles of the rotors which is the location of equal rotational speed on the rotors resulting in rolling contact and thereby in reduced or no sliding contact and thus less wear.
A substantial amount of end-running clearance must be maintained at the discharge end of screw compressors in order to prevent failure from rotor seizure. Seizure may be caused by the thermal expansion of the rotor or by the intermittent contacts between the rotors and the end casing due to pressure pulsations in the compression process.
It is an object of this invention to reduce leakage in a screw machine.
It is another object of this invention to relax machining tolerances without increasing leakage.
It is a further object of this invention to reduce oil sealing requirements in screw machines.
It is an additional object of this invention to minimize the power loss due to friction and to prevent wear. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
In accordance with the present invention, a coating is applied to one or more portions of the screw rotors and/or the inner bore surfaces of the housing.
In one aspect of the present invention, a low friction, wear resistant material may be deposited on the rotor tip where the rotors can have nominal contact with the housing as well as normal contact with each other. The rotors coact with each other, in pairs, as well as with the housing. While tight machining tolerances reduce the leakage due to these coactions between the rotors themselves and also with the housing, other things can be done in conjunction with the tight tolerances or in lieu of tight tolerances. Examples of suitable low friction, wear resistant coatings include multi-layer diamond-like-carbon (DLC) coating, titanium nitride and other single material, single layer nitride coatings, as well as carbide and ceramic coatings having both high wear resistance and a low coefficient of friction.
In another aspect of the present invention, conformable coatings may be located on the inner bore surfaces of the housing and/or in the rotor valleys. Examples of suitable conformable coatings include iron phosphate coating, magnesium phosphate coating, nickel polymer amalgams and other materials that yield elastically when a force is applied. Placement of conformable coatings on the inner bore surfaces of the housing and/or in the rotor valleys can reduce leakage and oil sealing requirements while relaxing manufacturing tolerances.
A surface coated or otherwise equivalently treated with such a low friction, wear resistant material is more forgiving to sliding contact than is an untreated surface. There also exists a synergistic effect associated with such a treatment in that the coated surface has a greater tolerance to sliding contact. In accordance with a further aspect of the present invention, this allows the contact band to be moved further away from the pitch circle, thus further reducing the contact force and reducing the overall wear potential over even the treated rotor with a relocated contact band. Locating the contact band near the pitch circles of the rotors is the conventional practice, as noted, and represents the desire to have nearly pure rolling contact.
The location of the contact band is a design feature and can be removed from the pitch circle or otherwise located where you wish. By moving the contact band away from the pitch circle the loading between the rotors can be reduced and this is particularly important for a female rotor drive. As contact starts to move away from the pitch circle there is more sliding contact rather than pure rolling contact. The blow hole area, which refers to the leakage area defined by the meshing rotor tips and the edge of the cusp between adjacent bores of a screw machine, can only be reduced to zero if the respective pitch circles correspond to the root circle of the male rotor and the tip circle of the female rotor. This necessarily requires the contact band to be located away from the pitch circle in response to trade-offs between the transmission angle, contact pressure, machineability of the root radius of the male rotor, and the amount of sliding that will take place.
The penalty for maintaining this large end-running clearance is to increase the leakage from the high pressure zone into the low pressure zone. In accordance with a further aspect of the present invention, by applying a wear resistant coating having a low coefficient of friction at the end face of the rotors or at the surface of the end casing or by inserting a coated piece between the rotor ends and the end casing, the end-running clearance can be reduced at least by 50%. The compressor performance is improved due to the reduced leakage at the discharge end.